(1) Technical Field
The present invention relates to semiconductor manufacturing. More specifically, the invention teaches an improved fluid application systems for stripping photoresist from silicon wafers, particularly wafers having dense top metal circuit patterns with sidewalls which obscure residual photoresist, and the like.
(2) Description of the Prior Art
The following four documents relate to methods dealing with stripping and cleaning of substantially planar objects.
U.S. Pat. No. 4,251,317 issued Feb. 17, 1981 to Foote, shows a gas bubbler in combination with a wafer cassette rotation within a wet wafer cleaning tank.
U.S. Pat. No. 5,868,898 issued Feb. 9, 1999 to Liu et al., shows a wet wafer cleaning tank with a fluid distributor to agitate the stripper and form bubbles.
U.S. Pat. No. 5,704,981 issued Jan. 6, 1998 to Kawakami et al., recites a buffer plate for distributing gas in a reactor.
U.S. Pat. No. 5,954,885 issued Sep. 21, 1999 to Ohmi, shows a cleaning method using wet tanks and ultrasound.
U.S. Pat. No. 5,464,480 issued Nov. 7, 1995 to Matthews, shows a gas diffuse for a organic stripping/cleaning tank.
During the forming of integrated circuits on semiconductor wafers, several process steps require submersing the wafers in liquid chemicals contained in an immersion tank is generally a practical high-throughput, flexible fabrication process. Examples include, chemical etching, photoresist stripping, and wafer cleaning. In a typical wet chemical process tank, in order to ensure a perfect mix of acids, detergents and the like or a good uniformity in the acid itself, constant stirring or agitation of the solution in the tank is desired. While mechanical stirring or agitation techniques have been used, the moving components of a stirrer frequently generate contaminant particles that are detrimental to the wafer surfaces. An example of a chemical process system equipped with a bubbler and a mechanical cassette rotating apparatus is shown in FIG. 1.
The apparatus illustrated schematically in FIG. 1 is of the prior art. As shown in that figure, is a tank 10 which contains liquid etchant 12 in the form of an acid bath. The tank has a mechanism 20 mounted thereto, wherein a housing 22 is rotatably mounted and driven with a sprocket and chain combination 30 by a motor (not shown). A cassette 40 containing wafers 42 is supported therewithin housing 22. The housing 22 and cassette 40 have appropriate openings therein so that etchant 12 may reach the wafers 42. A manifold 54 running transversely and adjacent the bottom of the tank 10. The longitudinal axis of the manifold 54 being substantially parallel to and directly below the axis of rotation of the housing 22. The manifold 54 defines a plurality of evenly-spaced openings along the upper surface thereof. Nitrogen gas is supplied through tubes 52. The gas bubbles 60 travel in a direction 62 generally perpendicular to the axis of rotation of the cassette 40.
The fabrication of integrated circuits on a semiconductor wafer involve a number of steps where patterns are exposed through lithographic photomasks into a photosensitive resist covering the wafer. After developing, open areas in the photoresist permit subsequent processes such as inclusion of impurities, oxidation, etching, and metalization to be performed. The photoresist is thereafter stripped from the wafer following each of the aforementioned process steps. From a chemical reaction point of view, conventional methods for stripping photoresist have low reaction rates due to the low collision frequency between the stripper and photoresist molecules. Furthermore, from a thermodynamics point of view, the activation energy of photoresist molecules dissolving into the stripper is intensified under static conditions, thus leaving residues on the wafers in view of the fact that an incomplete chemical reaction takes place in a limited processing time. The process suffers the risk of re-depositing the dissolved impurities onto the wafer in a viscous static stripper due to the low solubility of the photoresist.
During latter stages of circuit fabrication, the forming of various metal conductive layers transform the wafer's topographic surface into a three dimensional maze of metal circuit lines that are closely separated, thereafter, making the circuit line's sidewall height proportional in size to its width and spaces. The process of stripping the photoresist from between these three dimensional circuit lines becomes increasingly challenging, particularly on the 6 and 8 inch diameter wafers. Residues left behind are harmful contaminants to the microscopic circuits which the fabrication process creates.
The apparatus of FIG. 1, having a rotating mechanism immersed in the process chemical, conventionally used when etching wafers, is not recommended for use during photoresist stripping or for cleaning wafers because of particulate generated by the cassette rotating drive members, hence, contaminating the process chemicals and also the wafers being processed.